(1) Field of the Invention
The present invention is applicable to a vehicular navigation system and relates generally to a system and method for recognizing the running environment of a vehicle, whether the vehicle is running on an elevated freeway, a regular highway or ordinary streets.
(2) Background of the Art
Various types of systems for providing running environment information for a vehicle driver have recently been proposed. One of the above-described systems is a navigation system.
One of the previously proposed navigation systems is exemplified by Japanese patent application First Publication No. (Showa) 63-109319 published on May 14, 1988.
The navigation system disclosed in the above-identified Japanese patent application First Publication estimates the position of the vehicle whenever the vehicle has passed through an intersection on the basis of data on the running environment, i.e., mapped data on roads, intersections, distances between the intersections, and orientations, and directional data detected by a geomagnetic sensor and displays the data on a map on a screen of a display unit. The above-mentioned system also determines a route to a predetermined destination on the basis of this data.
The above-described navigation system, when put into practice, uses a flat plane (two-dimensional) to display the mapped data described above to provide information on the generally complicated road network found chiefly in urban areas.
However, taking the recent road environment into account, since elevated freeways are often built above city streets, and may in fact parallel some main streets, accurate data mapping becomes very complicated. To optimize such a navigation system it is desirable to display simultaneously ground level streets and elevated freeways in order to provide the driver with accurate running environment data and enable accurate identification of these roads.
To achieve the above-described and other objects, three dimensional data preparation in addition to the flat plane data described above has been considered.
However, in a case where three dimensional data is modeled by the previously proposed navigation system the following problem will occur.
That is to say, since the data on ground-level roads and data on elevated roads (freeways) are handled at the same level, the three-dimensional data is treated as two-dimensional data. Therefore, these roads are displayed on the same line based on the scale of the map. In a case where the ground level road and elevated freeway run parallel to each other, both cannot be accurately identified. In addition, when one or both roads branch from a parallel state, a deceptive intersection appears at the branching point above. Therefore, increased processing time results due to unnecessary route determining operations and the possibility of generating erroneous route and/or position data is increased.
On the other hand, if the above-described data is not handled simultaneously, i.e, each type of data is used separately according to necessity, the problem will be solved. In this case, extremely accurate identification between ground level roads and elevated freeway is prerequisite, and, an extremely important problem is introduced in that if the system provides deceptive information, subsequent route and position estimation will become impossible after an erronous recognition is carried out.